As dies for thread rolling screws, there are flat dies, round dies, planetary dies, etc. The cutting-edge surface of each die has a complementary shape of a shape of an object to be formed. In the case of thread rolling a screw, the rolling cutting-edge surface has projections of a triangular cross section, with an inclination provided according to a lead angle.
A general rolling die for thread rolling is constructed of a material having hardness of about Rockwell HRC60 in at least the rolling cutting-edge surface, by thermally treating a tool steel or an alloy tool steel.
As the rolling cutting-edge surface has high hardness, generally the cutting-edge surface is finished by grinding with a grindstone of a formed shape. However, as the surface coarseness becomes fine by the grinding, when a rolling die is driven, a phenomenon that a thread-rolled material slides along the rolling die occurs due to a friction between the die and the material at the time of an inverted rotation of the material. Therefore, the rolling cutting-edge surface is roughened by shotblasting, shotpeening or the like after the grinding.
As explained above, it is general that a manufacturing of a rolling die involves many processes such as a formation of a rolling cutting-edge surface by thermal treatment and grinding, and a post-processing (the processing of making the surface rough).
Recently, along with the progress in high-class materials such as automobile parts and other safety bolts, there has been an increase in the hardness of the materials. This is attributable to a severer regulation on the specification of the quality of bolt strength and social requirements for improved strength for performance improvement, etc.
As the hardness of bolt materials increases, the life of the rolling die for thread-rolling screws is necessarily lowered. This makes it necessary to frequently replace rolling dies and regrind the dies after the replacement. Therefore, it is not possible to secure sufficient life of the rolling dies based on the above-described manufacturing method. This has come to lower the productivity and increase costs. Thus, the improvement in bolt strength has come to bring about cost increase more than is necessary.
Conventionally, as a measure for solving this type of problem, the maintenance of necessary strength has been carried out based on selection of materials, thermal treatment, etc. However, under the environment of a further increase in the hardness of materials, it has come impossible to achieve further improvement in the strength of the rolling dies according to the conventional method.
As surface processing methods for improving the strength, there are methods of processing a hard film onto the cutting-edge surface of the die by deposition plating such as CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition), etc. However, according to the deposition plating such as the CVD (Chemical Vapor Deposition) or the PVD, the material is treated in a high-temperature furnace during a predetermined period. Even if the hardness is adjusted to a necessary hardness by thermal treatment, a tempered state occurs and the hardness is lowered and the strength is lowered. Therefore, this is not a practical measure.
It is also possible to increase the hardness and strength of the surface of the rolling die for thread-rolling screws and the whole die, by a surface hardening processing such as nitriding or cementation. However, such surface hardening processing has little effect in actual practice in the rolling processing that is a kind of forging where a very large compressive load and smoothing operate.
To cope with this situation, as disclosed in Japanese Patent Publication No. (Hei2-46940), there has been proposed a discharge hardening processing technique for shift dispersing electrode substances of WC, TiC, TaC, etc. onto a rolling cutting-edge surface of a rolling die by intermittently generating a spark discharge based on a contacting and separation between the discharge electrode and the rolling cutting-edge surface, in a state that a discharge voltage is applied between the rolling cutting-edge surface (formed surface) and the discharge electrode that is disposed opposite to the cutting-edge surface.
The surface of a hard film obtained by the above discharge hardening processing becomes in a state that discharge traces have been integrated. Thus, it has been known that a coarse surface is obtained as compared with the ground surface. By only carrying out the discharge hardening processing without carrying out the surface roughing processing such as the shotblasting and the shotpeening, it is possible to obtain the coarseness and hardness (abrasion-proof) that are necessary for the rolling die. It is also possible to omit the provision of fine vertical grooves called a serration into a bite-starting end of a material for the rolling die for ensuring a secure bite of the material.
However, the hard film obtained by the above discharge hardening processing has a too coarse surface. Further, a satisfactory inclination substance area cannot be obtained between the hard film and a base metal. Thus, there is a problem in the strength of the hard film.
Further, the surface coarseness and abrasion-proof required by the rolling cutting-edge surface of the rolling die are not uniform in the rolling direction (a direction from a material bite-starting end toward a rolling finishing end). Naturally, the required surface coarseness and abrasion-proof become maximum at the material bite-starting end where a large processing load is applied. The required surface coarseness and abrasion-proof gradually become lower toward the rolling finishing end.
Regarding the above aspect, according to the conventional rolling die, the hard film is either uniformly provided on the whole surface of the rolling cutting-edge surface of the rolling die or is provided on only a part of the surface. The surface coarseness and the thickness (abrasion-proof) of the hard film formed on the rolling cutting-edge surface are uniform.
Therefore, when the hard film is provided on the rolling cutting-edge surface of the rolling die to have the surface coarseness and thickness that are required at the material bite-starting end, the surface coarseness and thickness are too much at the rolling finishing end. Thus, there arises a waste in the discharge surface processing material and the processing time. Further, when the surface coarseness at the rolling finishing end is the surface coarseness that is required at the material bite-starting end, there is another inconvenience that the surface coarseness of the thread-rolled product becomes coarse.
The present invention has been provided to eliminate the above problems. It is an object of the invention to provide a rolling die and a surface processing method for a rolling die that provide a formation of an excellent hard film on the rolling cutting-edge surface to have high-level hardness and abrasion-proof as a rolling tool, thereby to achieve a long-life rolling tool, without the need for a frequent replacement of dies for manufacturing high-strength bolts, by providing surface coarseness necessary for a rolling die without a special post-processing, and obtain material slide-prevention effect and coarseness of a manufactured product, a reduction in manufacturing cost and a long life of the manufactured product.